Agglomerates containing iron and at least one further element of groups 5 or 6 of the periodic system

ABSTRACT

The invention relates to agglomerates containing iron and at least one further element of groups 5 or 6 of the periodic system, characterized in that they have a porosity in the range of 20 to 65% by volume, in particular of 30 to 45% by volume. Hereby, a rapid dissolubility of the agglomerates in a metal melt is achieved.

[0001] The present invention relates to agglomerates containing iron and at least one further element of groups 5 or 6 of the periodic system, their use, and a method for producing them. As further element, one may in particular consider molybdenum and tungsten.

[0002] From DE-A-196 22 097 agglomerates are known which are formed of an iron/molybdenum alloy having 60 to 80% by weight of molybdenum and are used as alloying agents for metal melts containing iron and molybdenum.

[0003] Molybdenum is used f.i. as an alloying element for producing high-strength structural steels containing molybdenum, alloyed cast iron types as well as molybdenum-containing, rust-resisting, acid-resisting and heat-resisting steels and nickel base alloys.

[0004] When producing molybdenum-containing alloys, steels and cast iron types, for economic reasons the greater part of the necessary molybdenum alloying contribution is added to the melt either in the form of revert scrap containing molybdenum or in the form of briquetted molybdenum trioxide (MoO₃).

[0005] Adding molybdenum in an oxidic form is possible because in liquid steel the iron acts as a reducing agent and thus, the MoO₃ is transformed into metallic molybdenum. However, this way of adding molybdenum is difficult in terms of manipulation. Attention has to be paid to a deep penetration of the MoO₃ into the melt, given that at the temperatures of the liquid steel MoO₃ vaporizes very easily and/or is set in the slag, whereby insufficient immersion of the MoO₃ may cause great losses in yield.

[0006] In the course of a so-called secondary metallurgical aftertreatment, finishing the smelting of the above steels, for reducing the detrimental gas contents (oxygen, nitrogen), for the exact setting of the desired casting temperature and the final analysis of the steel, the fine setting of the molybdenum content is therefore effected with lumpy so-called ferromolybdenum.

[0007] Ferromolybdenum is an iron/molybdenum alloy usually having 60-80% by weight of molybdenum and produced by way of a metallothermal process. The metallothermal production according to the thermite burning process is complex, given that the metals iron and molybdenum have to be melted on and together. The use of expensive reducing agents such as aluminium or ferrosilicon is required. The process may be automated only to a limited extent. This results in a higher market price of the ferromolybdenum as compared to the molybdenum trioxide (MoO₃).

[0008] A disadvantage of ferromolybdenum produced according to the thermite process is the relatively high lump density (fi. about 8.8 g/cm³ in standard FeMo70), resulting in that when alloying f.i. steel melts (density about 7.5 g/cm³), the material sinks to the bottom of the melting vessel where it forms depositions difficult to dissolve which only come off in the subsequent melts. Dissolving such ferromolybdenum lumps in the liquid steel bath is additionally made more difficult by the high melting point of the material, which in the case of a usual commercial FeMo70 quality is about 1950° C. The temperatures in the steel bath are significantly below this level so that now the dissolution of the FeMo parts can only be effected by way of diffusion processes which, accordingly, require long periods of time.

[0009] The dissolution of ferromolybdenum produced according to the thermite process is basically carried out according to the following mechanism:

[0010] The alloy lumps immerging into the liquid melt sink to the bottom of the treater. This is caused by the high density of the parts, which is higher than that of the liquid steel. An outer layer of solidified steel forms on the lumps, which layer results from the quench effect of the immerged cold FeMo lump. Due to the heat transition from the melt to the alloy lump, the outer layer subsequently gets dissolved again. However, given that the melting point of the alloy lumps is above the temperature of the liquid steel bath, the alloy lumps can only dissolve by diffusion of iron from the steel bath into the boundary layer of the melt and the alloy lump and by the reduction of the melting point associated therewith.

[0011] According to the above DE-A-196 22 097, agglomerates are produced from an iron/molybdenum blend by briquetting, wherein the iron/molybdenum blend is obtained by reducing a fine-grained molybdenum-trioxide/iron-oxide blend with hydrogen-containing gas. Briquetting is carried out by adding a binding agent such as water glass in order to improve the grain binding. Agglomerates having a lump density higher than 3.5 g/cm³ are formed therein.

[0012] Disadvantages of this process are on the one hand the use of binding agents which introduce detrimental tramp elements such as silicon, sulfur, and hydrogen into the steel, and on the other hand the poor lump densities and resistances of the material which are available with this method and which lead to great losses of molybdenum to the slag.

[0013] U.S. Pat. No. 5,954,857 describes the production of briquets consisting of molybdenum oxide with NaOH as binding agent. When introducing these briquets into liquid steel melts, the molybdenum oxide is reduced to the metallic molybdenum by the liquid iron, wherein iron oxide is formed. Disadvantages of this process are the danger of losing molybdenum oxide by absorption in the slag which is on the surface of the liquid steel, and the losses of iron occurring in the reduction of the molybdenum oxide.

[0014] From U.S. Pat. No. 4,400,207 a method for producing metal alloys is known according to which molybdenum oxide, f.i., is mixed with a fine ferrosilicon powder in the stoichiometric ratio. As a binding agent, up to 5% bentonite are admixed, and the mixture then is briquetted. When introducing these briquets into steel melts, the contained ferrosilicon acts as a reducing agent for the molybdenum oxide which passes over to the steel melt in a metallic form.

[0015] A disadvantage thereof is the formation of silicon oxide as a reaction product which has to be set in the slag, which in the steel-making processes used today is only possible when taking additional measures.

[0016] The invention has as its object to provide agglomerates containing iron and at least one further element of groups 5 or 6 of the periodic system and having an improved dissolubility in metal melts, in order to keep the costs of treating the melt low. In particular, the agglomerates should not sink to the bottom of a metal melt and should have, furthermore, a sufficient resistance in view of storage and transport. Moreover, the quality of the metal melt should not be prejudiced by tramp elements being in the agglomerate and acting as binding agents, fi., and a loss of molybdenum and iron should be avoided.

[0017] According to the invention, this object is achieved insofar as the agglomerates have a porosity in the range of 20 to 65% by volume, particularly of 30 to 45% by volume.

[0018] The agglomerates according to the invention have a porosity and, by that, a lump density which on the one hand allows the penetration of a slag cover on a metal melt and allows the agglomerates to penetrate into the metal melt. On the other hand, the inventive porosity of the agglomerates results in that capillary action fills the pores of the agglomerates with metal melt and in that the thereby occurring enlargement of the boundary surface between the metal melt and the agglomerate rapidly dissolves the regions filled with metal melt. Here, dissolving means the melting of the agglomerates and the homogeneous distribution of the components of the agglomerates in the metal melt.

[0019] The dissolution process of the inventive agglomerates in a metal melt can be described as follows:

[0020] After penetration of the agglomerates through the slag cover on the melting bath and immersion into the melt, a boundary layer of solidified steel forms on the surface of the agglomerates, which steel results from the quench effect of the cold agglomerates. This boundary layer is much thinner than the layer that forms when using ferro-alloys produced with the thermite process, given that the thermal capacity of the agglomerates is lower due to high porosity.

[0021] Even though the density of the agglomerates is lower than that of the liquid steel, they immerge deep into the melt because of the kinetic energy of the parts which have to cover a corresponding height of fall before impinging on the steel bath.

[0022] After dissolution of the outer zone, the liquid steel penetrates into the pores of the agglomerates. The thereby produced large boundary surface between the agglomerate and the melt leads to a rapid warming and diffusion of iron in this boundary layer, which eventually causes the dissolution of the agglomerates. In addition, the gas included in the pores of the agglomerates expands because of the rapid warming and enters into the metal melt. The thereby generated turbulent flow on the surface of the agglomerates causes the rapid reduction of the existing concentration gradients on alloying agents between the boundary surface and the melt, which leads to an increase in the diffusion rate that depends, according to Fick's law, on the concentration gradients.

[0023] A high dissolution rate means the saving of time and costs in the production of alloyed metal melts.

[0024] According to a preferred embodiment, the inventive agglomerates contain as further element molybdenum in an amount of 45 to 85% by weight, preferably of 60 to 80% by weight. The lump density of these agglomerates is preferably 4.2 to 6.3 g/cm³, particularly preferred 4.5 to 5.7 g/cm³.

[0025] According to another preferred embodiment, the agglomerates contain as further element tungsten in an amount of 60 to 90% by weight, preferably of 70 to 85% by weight. Their lump density is preferably 4.7 to 8.4 g/cm³, particularly preferred 5.8 to 7.4 g/cm³.

[0026] The present invention also relates to the use of the agglomerates for producing alloyed metal melts, especially molybdenum-alloyed and/or tungsten-alloyed metal melts.

[0027] The invention further relates to a process for producing the agglomerates, wherein iron oxide and oxides of at least one further element of groups 5 or 6 of the periodic system are reduced to the respective metals.

[0028] U.S. Pat. No. 3,865,573 relates to a process for producing molybdenum powder and/or ferromolybdenum, wherein molybdenum oxide and/or a blend of molybdenum oxide and iron oxide are reduced in a two-stage fluidized-bed process.

[0029] U.S. Pat. No. 4,045,216 describes a process for producing directly reduced molybdenum-oxide pellets, based on the two-stage reduction of molybdenum-oxide pellets in an hydrogen-containing atmosphere. As reduction aggregate, a shaft furnace is used which is traversed in counterflow by the product and the reducing gas. In this process, pellets having a very low density and abrasion resistance are produced.

[0030] The process according to the invention is characterized in that the reduced metals are compacted, especially briquetted, without adding any binding agents and in that the thereby formed compacted products are sintered.

[0031] Sintering is effected preferably at temperatures from 1000 to 1400° C., in air or preferably in an inert-gas atmosphere, for 15 to 60 minutes. At the inventive sintering temperatures, mainly the iron contained in the agglomerates acts as sinter-active phase and as a binder for the particles contained in the agglomerates. Thereby the agglomerates are prevented from becoming too dense during the sintering process, which would have a negative effect on their dissolution in metal melts.

[0032] In the following, the invention will be explained in more detail by means of three exemplary embodiments and FIGS. 1-6.

EXAMPLE 1

[0033] A powder mixture consisting of 74% molybdenum, 21% iron and 5% oxidic contaminations such as silica, aluminium oxide and calcium oxide and produced by reducing a mixture of oxides of technical purity of both metals in an hydrogen atmosphere was compacted to agglomerates having a diameter of 60 mm and a height of 40 mm in a compacting press.

[0034] These pressed parts were sintered for different periods of time in a laboratory sintering furnace in a nitrogen atmosphere at 1170° C. After having cooled the parts and withdrawn them from the sintering furnace, samples were taken from the parts, and the porosity was measured.

[0035] Table 1 below shows the porosities of FeMo agglomerates as a function of the sintering period and the resulting lump density. Here, the porosity was measured with an Hg porosimeter. By comparison, the density and porosity of a conventional FeMo agglomerate is indicated (Comparative Example). TABLE 1 Sintering period at 1170° C. Density [g/cm³] Porosity Sample 1 15 4.15 42.4 Sample 2 25 4.3 39.7 Sample 3 45 5.48 23.1 Sample 4 60 6.0 — Comparative Example 8.0 0

[0036]FIG. 1 shows the pore size distribution of FeMo agglomerates produced with the process according to the invention. The particle size of the agglomerates was in a range of 2 to 4 mm. The measurements were taken by means of an Hg porosimeter at an Hg column pressure of 200 mm.

[0037] The curve numbered 1 represents the pore size distribution of the FeMo agglomerates referred to as sample 1 in the above table after sintering at 1170° C. The molybdenum content of these agglomerates was 74%. The curve numbered 2 represents the pore size distribution of the FeMo agglomerates according to sample 2. Finally, the curve numbered 3 represents the pore size distribution of the agglomerates according to sample 3. It can be seen from this that the mere choice of different sintering parameters (temperature and period of time) makes it possible to vary the number of the pores and the distribution of the pore size within a wide range.

[0038] Agglomerates produced according to the inventive method and corresponding to the material referred to as sample 1 in table 1 were dissolved in a steel melt in a laboratory electric-arc furnace (see Example 2).

[0039]FIG. 2 shows, in an exemplary manner, the dissolution rate of an inventive FeMo agglomerate as compared to standard FeMo (produced by way of a silicothermal process). The curves were recorded when smelting a high-speed-steel quality (S-6-5-2, 1.3343) with a molybdenum content of 5%. The composition of the steel produced in the experiment is indicated in table 2 below. TABLE 2 S-6-5-2, 1.3343 % by weight C 0.9 Cr 4.1 Mo 5 V 1.8 W 6.4 Fe rest

[0040] Data Specification of the Experimental Electric-arc Furnace:

[0041] Electrical data: 3-phase; power max. 200 kw voltages: 52/63,5/75/86,5/90/110/120/150 v

[0042] Electrodes: graphite Ø100 mm, automatic control

[0043] Furnace crucibles: infeed with magnesite, with a casting nose effective volume about 100 1

[0044] The size of the experimental melt was 300 kg. The melt was used in a three-phase electric-arc furnace as a set-up charge, that is, the steel composition was set to a pure-iron melt by adding ferro-alloys in a corresponding amount. As a first step, all of the alloying elements except Mo were added and set according to the target analysis. For protection against reoxidation, the steel bath was covered with a calcium aluminate slag.

[0045] In a first experimental melt, the molybdenum content was set by adding ferromolybdenum having a grain size of 5-50 mm and produced according to the thermite process. After having added the FeMo, samples were taken from the melt at short intervals. A second melt was produced in the same way except that here, the inventive agglomerates were used to set the molybdenum content. It could be seen that the inventive agglomerates (represented in FIG. 2 by the broken line) dissolved much faster than standard FeMo (represented in FIG. 2 by the unbroken line).

[0046] The significant advantage of the agglomerates according to the invention is that they dissolve faster in steel melts than standard FeMo, which results in the saving of time and, by that, costs for the user.

EXAMPLE 2

[0047] In a large-scale application experiment, the dissolution behaviour of the inventive agglomerates was compared with that of usual commercial ferromolybdenum produced according to the thermite process.

[0048] Agglomerates produced with the inventive method and corresponding to the material referred to as sample 1 in table 1 were dissolved in a steel melt in a steel ladle having a charge weight of about 190 t, and the dissolution rate was compared to that of ferromolybdenum produced according to the thermite process. Table 4 indicates the composition of the produced steel. TABLE 4 Element % by weight C <0.2 Si 0.1 Mn 1.2 Cr 0.25 V 0.02 Mo 0.5

[0049] During the experiments, the steel bath was protected against reoxidation by a calcium aluminate slag, and for better homogenization, the melt was washed with Ar by means of a fireproof lance introduced from above into the melt.

[0050] A total of six experiments was carried out, two charges of them with usual commercial ferromolybdenum having a grain size of 5-50 mm and four charges of them with the agglomerates according to the invention. The alloying agent was added via a slide from a bunker system. The samples were taken by means of an automated sublance system at intervals of about 20 s.

[0051] The experimental parameters are summed up in table 5. TABLE 5 39999 40000 40300 40301 40324 40348 Charge Number FeMo St. FeMo St. Aggl. 1 Aggl. 2 Aggl. 3 Aggl. 4 LD converter LD tapping time 11:24 12:16 12:46 13:35 11:16 09:34 Mo content in LD % 0.064 0.074 0.012 0.066 0.075 0.087 Calc. weight LD t 190.8 184.2 192.8 182.7 192.8 189.9 TN Temp. arrival ° C. 1616 1627 1628 1604 1640 1627 FeMo addition kg 1000 1000 1000 1000 1000 1000 Mo content kg 681.7 681.7 724 703 743 743 Gas stirring time min 17 14 14 14 15 15 Gas flow de-S NI/min 925 922 763 765 900 922 Continuous casting plant Start cont. casting time 13:28 14:31 15:07 16:09 14:00 15:48 Charge weight t 191.1 181.7 191.7 183.1 190.1 192.3 Mo content 1 % 0.493 0.622 0.48 1 0.497 0.629 0.49 Mo yield 1 % 98.6 94.1 96.1 99.9 95.8 95.6 Mo content 2 % 0.488 0.637 0.482 0.501 0.625 0.492 Mo yield 2 % 97.4 96.7 96.3 100.8 95.1 96.1

[0052] As to FIG. 3, it can be seen that the inventive agglomerates dissolve much faster, yielding more molybdenum. From the curves relating to standard FeMo it can be seen that even after periods of treating the melt of about 10 min, less than 80% of the added molybdenum has dissolved in the melt. In practice, this means that such a melt has to be heated up once again in a pan furnace in order to obtain a commercial molybdenum yield, which, however, requires higher treatment costs.

EXAMPLE 3

[0053] Agglomerates produced with the inventive method and corresponding to the material referred to as sample 1 in table 1 were dissolved in a steel melt in a steel ladle having a charge weight of about 90 t, and the dissolution rate was compared to that of ferromolybdenum produced according to the thermite process.

[0054] Table 6 indicates the chemical composition of the produced steel. TABLE 6 Elements % by weight C 0.02 Si 0.5 Mn 1.5 P <0.04 S <0.0055 Cr 17 Ni 11 Mo 2.0 Al <0.007 N₂ <0.03

[0055] Four charges of the steel, each having a melting weight of about 90 t, were produced. In the ladle washing station, FeMo produced according to the thermite process was added to two charges and the inventive agglomerates were added to two charges. The added quantities can be seen in table 7. After the addition, samples were taken from the melt at regular intervals so as to be able to examine the increase in molybdenum content. TABLE 7 FeMo addition Experiment [kg] Form of FeMo E1 347 standard E2 414 standard E3 250 agglomerates E4 350 agglomerates

[0056] Additionally, slag samples and samples from the cold rolled strip produced from the steel were taken during the experiments so as to be able to study a possible effect on the degree of purity of the produced steel, caused by the use of the inventive agglomerates.

[0057]FIG. 4 shows a comparison of the dissolution rates of the ferromolybdenum produced according to the thermite process vs. those of the inventive agglomerates. It can be seen that also in Example 3, the inventive agglomerates dissolve faster in steel than the standard FeMo.

[0058] The examinations of the degree of purity of the product produced did not show any significant changes caused by the use of the inventive agglomerates for producing molybdenum-alloyed steels.

[0059] Referring to exemplary applications in steel melts, FIGS. 5 and 6 show further examples of the dissolution rates of inventive FeMo agglomerates as compared to standard FeMo. 

I claim:
 1. Sintered agglomerates containing iron and at least one further element of groups 5 or 6 of the periodic system, prepared by a process comprising the steps of (i) reducing an iron oxide and an oxide of at least one further element selected from the group consisting of elements of groups 5 and 6 of the periodic system to their respective metals, (ii) compacting, in the absence of a binding agent, the reduced metals prepared by step (i), and (iii) sintering the compacted product of step (ii), wherein the sintered agglomerates have a porosity in the range of 20 to 65 percent by volume.
 2. Sintered agglomerates according to claim 1 which contain, as a further element, molybdenum in an amount of 45 to 85 percent by weight.
 3. Sintered agglomerates according to claim 2 having a lump density in the range of 4.2 to 6.3 g/cm³.
 4. Sintered agglomerates according to claim 1 which contain, as a further element, tungsten in an amount of 60 to 90 percent by weight.
 5. Sintered agglomerates according to claim 4 having a lump density in the range of 4.7 to 8.4 g/cm³.
 6. A method of producing alloyed metal melts comprising adding, to a metal melt, sintered agglomerates according to claim
 1. 7. A method of producing alloyed metal melts comprising adding, to a metal melt, sintered agglomerates according to claim
 2. 8. A method of producing alloyed metal melts comprising adding, to a metal melt, sintered agglomerates according to claim
 3. 9. A method of producing alloyed metal melts comprising adding, to a metal melt, sintered agglomerates according to claim
 4. 10. A method of producing alloyed metal melts comprising adding, to a metal melt, sintered agglomerates according to claim
 5. 11. A method for producing sintered agglomerates of claim 1, comprising the steps of (i) reducing an iron oxide and an oxide of at least one further element selected from the group consisting of elements of groups 5 and 6 of the periodic system to their respective metals, (ii) compacting, in the absence of a binding agent, the reduced metals prepared by step (i), and (iii) sintering the compacted product of step (ii).
 12. A method for producing sintered agglomerates of claim 2, comprising the steps of (i) reducing an iron oxide and an oxide of at least one further element selected from the group consisting of elements of groups 5 and 6 of the periodic system to their respective metals, (ii) compacting, in the absence of a binding agent, the reduced metals prepared by step (i), and (iii) sintering the compacted product of step (ii).
 13. A method for producing sintered agglomerates of claim 3, comprising the steps of (i) reducing an iron oxide and an oxide of at least one further element selected from the group consisting of elements of groups 5 and 6 of the periodic system to their respective metals, (ii) compacting, in the absence of a binding agent, the reduced metals prepared by step (i), and (iii) sintering the compacted product of step (ii).
 14. A method for producing sintered agglomerates of claim 4, comprising the steps of (i) reducing an iron oxide and an oxide of at least one further element selected from the group consisting of elements of groups 5 and 6 of the periodic system to their respective metals, (ii) compacting, in the absence of a binding agent, the reduced metals prepared by step (i), and (iii) sintering the compacted product of step (ii).
 15. A method for producing sintered agglomerates of claim 5, comprising the steps of (i) reducing an iron oxide and an oxide of at least one further element selected from the group consisting of elements of groups 5 and 6 of the periodic system to their respective metals, (ii) compacting, in the absence of a binding agent, the reduced metals prepared by step (i), and (iii) sintering the compacted product of step (ii).
 16. The method of claim 1 1, wherein the reduced metals are compacted into a briquette.
 17. The method of claim 12, wherein the reduced metals are compacted into a briquette.
 18. The method of claim 13, wherein the reduced metals are compacted into a briquette.
 19. The method of claim 14, wherein the reduced metals are compacted into a briquette.
 20. The method of claim 15, wherein the reduced metals are compacted into a briquette. 